The Stovebolt Site Forums The Shop Area Engine Shop Backfiring and Dieseling

My Chevy engine backfires and diesels when I try to turn it off. Here is the set up. The car is a restored
1956 Chevy Belair that I recently completed. It has an inline 6 cyl. 250 motor in it (not a 235). I’m using a conventional distributor with points and an external coil. The motor was professionally rebuilt both the block and head at a machine shop. It has original intake and exhaust manifolds with a new NOS Carter YF carburetor. I have only starter it 5 or 6 times so far. It is nearly new in that regard.
The car starts without any problem. However, when I give it gas, it hesitates and sputters. As it warms up, it runs very rough and wants to die. I feather the gas petal and it will increase RPMs but when I back off, it starts to die. At low RPM I can hear some knocking. When I try to shut off the engine, it will not stop and begins to diesel. Sometimes it backfires through the carburetor.
What is most likely wrong? Is the timing off? Valves not adjusted correctly? Suggestions appreciated.
Thanks, Glenn

Dieseling is caused by too-high idle speed. Find out why the engine won't idle, fix it, and your dieseling problem will go away. If that "NOS" Carter carb is speaking Mandarin, it's probably the source of the problem. The "new" clones are made offshore, and they're mostly good for paperweights, at least until they get filled with gas the first time. Then they become a stinky paperweight. If you paid less than $200.00 for it, it's probably not an original equipment Carter YF.
Jerry

The motor idles at low RPM, not high. The Carter carb is USA made. I paid more than $200 and I'm sure it is NOS. It was also professionally rebuilt with new gaskets, etc. before I installed it. Could the problem be that the choke is not opening or closing properly? I'm going to pull the plugs today and see if they are fouled.

Check the intake / exhaust manifold seal at the head, 250's are a little hard to seal up and they can cause all kinds of running issues. While the engine is running ( idling ), use an open, non-burning, propane torch and wave it around all the ports at the head. Go around the top and up under the intake. If you have a leak, the engine will speed up due to extra fuel it pulled. For exhaust leaks, just use your hand and feel for hot air rushing out.
For testing, mark your distributor and head with tape and a marker, then turn it some and see how it acts, if it's worse, go the other way, if it's the same, put it back to the marks and move on.

At this point, you have checked air and spark and both working, so you are down to the carburetor. Be sure the vacuum advance is working, and be sure the mechanical advance is not stuck. Pull the distributor cap and give the rotor a twist, it should snap back into place each time. Be sure the PCV valve is new and work, and you have no other vacuum leaks.

99.9% of all 250's have hydraulic lifters, they are set like all hydraulic lifters, find zero lash and go 3/4 to 1 turn more of the lock nut. For us " racers " we go 1/4 to 1/2 turn to gain a few more rpm at high speeds { at least we think we do }. To find zero lash, rotate the push rod with one hand while tightening the nut with the other, when you have zero play between the rocker and push rod, you found zero lash. The lifter must be on the base circle of the camshaft, this is where the valve s are closed. There are many ways to set valves, but I like to set the first cylinder on the compression stroke, both valves are closed, and check lash. I then rotate the engine till the #2 cylinder is on compression and check it, then 3 and so on till all 6 are done.

To hear knocking at idle is not good!

That engine will have hydraulic valve lifters. It's possible your builder has the valve adjustment a little too tight, and the lifters are keeping the valves slightly off their seats as the engine warms up. Try backing all the rocker arm nuts off 1/4 turn, and run the engine again. There's also a remote possibility of some voltage feedback from the alternator to the ignition system keeping the coil energized for a short time after the switch is turned off. Try disconnecting the big charging wire at the back of the alternator and running just on the battery to see if the problem goes away.
Jerry

Something quick and easy to check…

Verify your wires are routed to the proper firing order. 153624. I don’t know if incorrect firing order would explain the dieseling, but it would the crummy running.

All of HRL’s and Joe H’s are good. Not trying to detract, just want to make sure all the basics are covered.

I have been working on my car, but it still is not running right. I can get it to start when it is cold, but within 1 minute as it warms up it begins to run rough and starts to backfire. I must constantly keep feathering the accelerator to keep it running and then it will just stop.
I have checked these things: The wiring firing order is correct, and wires are new. The plugs are not fouled, but I replaced all of them anyway and it still did not fix the problem. I found no vacuum leaks, and the compression is good. There is 10V at the coil. The plugs have spark. I put in a different known good MST hotter coil. I rest the pints gap a bit wider. I removed the YF single carb and opened it up and it is clean with no debris or gunk in it. I drained all the gas and put in fresh 93 octane fuel. The vacuum advance can is new and seems to be working. I have not been able to check the timing because the engine will not run long enough for me to be able to do that.
So, I am now wondering if either the fuel pump is bad and not pumping enough gas into the carb or the distributor is not creating a hot enough spark at the plugs. When I checked the plug spark on 3 different plugs, the spark was a “red” color and not the normal “blue” color. I’m wondering if changing to an HEI type distributor would be a better choice. That is my next step. If that fails, I’ll replace the fuel pump.
Any other suggestions? What’s odd is that when it’s cold, it seems to run okay for a short time. As soon as it heats up it runs very rough and then dies.

Bad condenser maybe?

Did you check the valve adjustment? If not, I'm out of this discussion! "Blue" sparks quit happening in the 1960's when non-metallic spark plug wires became standard.
Jerry

Classic symptom of a failing condenser. It’s extremely hard these days to get a good one. Contact Jon G on this forum and order one of his replacement condensers and never replace another one.

I had a similar problem with a straight six Mustang. After checking timing, timing chain slack, points, cap , rotor, plugs, carburetor, etc, I decided to put the 56 year old condenser back in. Voila!

As I soon learned, you can't count on new parts to work - even right out of the box in many cases.

The brand new NGK condenser with less than 1500 miles on it failed. I have since installed one of Jon Goodman's 200,000 hour condensers on every single vehicle in my fleet (with the exception of the Ford pickups that have electronic ignition).

https://www.stovebolt.com/ubbthread...native-condenser-update.html#Post1388813

When you took the carb apart, did you check to see if the accelerator pump diaphragm was still intact? What were your numbers when you did the compression test? When you say the advance can seems to be working, what do you mean?

TexApache I think in the 70's GMC had trouble with dieseling so bad they put on a solenoid to close the throttle plates completely so the engine would stop when you shut it off. It was a nightmare . In 68 when the fuel made the carbon so hard you couldn't wire brush it off of the valves was when that started. AIR AIR INJECTION REACTOR !!

Yes, I backed off the valve nuts 1/4 turn.

How does the engine run after loosening the valve adjustments? It might be necessary to go through the complete adjustment sequence to be sure someone hasn't made a major blunder while assembling the engine. Have you removed the pushrod side cover to see how far the plungers are recessed into the lifter body?
Jerry

The compression results: #1=152, #2=155, #3=152, #4=155, #5=155, #6=155. seems okay.
Now that I know how to check the dwell, I'll do that this weekend.
I understand what you are saying about the accelerator pump. I'll check with Jon Goodman about this as he rebuilt this carb for me recently.

As of yesterday, I may have fixed the problem. The motor now starts and idles fine. It revs up without hesitation also. I plan to drive it a bit today. I did 4 things since I last reported. First, I replaced the fuel pump. Second I bought and new condenser from Jon G. and installed it. Third, I had some dirty gas in the tank. When it pumped through the filter it was dark orange in color. It seems that all the dirty gas has been purged from the fuel line and pump and now I'm getting clear gas through the pump and filters. Fourth, I set the idle up a bit.

I am thinking the motor was trying to run to lean. Not getting enough fuel or good clean fuel. Maybe a bad pump?

OK? GREAT!!! Or the needle is stuck on the compression gauge.

What kind of fuel pump did you install? I recently had two Chinese jobs fail. One lasted a few hundred miles, the other didn't pump a drop straight out of the box. Both are Carquest brand. Not that it matters. I think everybody sells the same slave labor-made pumps under different brand names.

I sent my original pump with the glass bowl on top to Then&Now Automotive for a rebuild. So far, so good.

I purchased a Delphi fuel pump.

Unfortunately brand names don't mean much anymore.

Test the fuel pressure and fuel output.

How do I test the fuel pressure? Is there a gauge that I can rent from Autozone or some place?

Just buy a pressure/vacuum gauge. They're cheap, and they have a bunch of uses. No shop should be without one.

https://www.amazon.com/8milelake-Vacuum-Tester-Carburetor-Pressure/dp/B01GHD0NAK

Jerry

Well, my Belair was still not running right, wouldn't start and began backfiring again. After going back to basics and checking lots of things, it turns out that the timing was way off. Perhaps as much as 40-50 degrees. That was causing the starting difficulty, backfiring and engine running hot. The cause of the timing problem is a misaligned harmonic balancer. I set the motor on TDC and made a new alignment mark on the balancer and redid the timing. After doing that, I turned the key and the motor started immediately and runs very well.


Now the question becomes, why is the harmonic balancer that far misaligned? Could I have the wrong HB on the motor. The motor is I6 250 and maybe the HB is for a 235? Would a 235 HB fit on a 250? Or could the HB have slipped on the shaft? Sheared off the key pin? I'm going to pull this current one off and replace it and want to get the correct new one.


Thanks again to everyone for the help.

Tex, you are a very lucky man. You have mentioned twice now that you are working on a car and we’re still talking to you. Some nice guys here.

250's drive the outer ring of the harmonic balancer with a bonded rubber ring. That outer piece also contains the grooves for the fan belt(s), and they're notorious for breaking the bond and letting the balancer ring slip. I guarantee it will happen again, no matter how many witness marks you make. Change the balancer- - - -they're dirt cheap on Ebay.
Jerry

I6 250 will have a keyed shaft and crank end bolt. I6 235 is press fit.
You are a lucky man indeed.
A loose HB could slip off the crankshaft and put you in a casket.

The outer ring can still come off, center bolt or not. There's nothing but glue holding it on, and once the bond gets broken, they can slip off without any warning. The hub stays on the crankshaft snout.
Jerry

That happened to me in high school on a '66 Mustang 6 cyl. I heard a loud bang and clanking, then my alternator gauge went all the way to discharge.

When I opened the hood I had a hard time figuring out what happened immediately because the belt was still in place - just no pulley groove on the crankshaft. It then dawned on me what had occurred. Amazingly the pulley ring didn'tt do any other damage under the hood when it fell off.

I ended up buying an entire '67 Mustang just to get the HB pulley off the engine. My brother took the body and swapped another drive train into it.

Just a reminder: it's not a "balancer" regardless of what the literature says.

Thanks everyone. I'm going to get a new balancer and replace the current one.

OK, let's use the proper technical term- - - -"Change that round thingy at the front of the engine!" Good grief, Charley Brown!
LOL!
Jerry